When a cell phone battery loses its charge, the cell phone shuts down and cannot be turned on until its battery is sufficiently re-charged. For many cell phones, even if the phone is connected to an external power source, such as a charger, the phone still cannot be turned on until its battery is sufficiently re-charged.
Depending on circumstances, inability to turn on and use a cell phone after shut down may result in various kinds of damaging consequences, including monetary loss, illness, trauma and even life-threatening danger. It would certainly be advantageous to mitigate this problem.
Regarding power management, in addition to cell phones, many mobile electronic devices such as notebook and laptop computers, personal data assistants (PDAs), digital cameras, and music and video players, can be powered by internal batteries or by external power sources. The internal battery, or battery pack, of such a device provides a DC source of power, and enables the device to operate in a standalone mode. Alternatively, such a device may be connected to an external AC power source, which is transformed to a DC voltage using a suitable power adaptor. In addition, often such a device may be connected to a host computer, and is able to use the power from the host as a DC power source.
Generally, when such a device is connected to an external AC power source, or to a host computer, the voltage coming into the device may be used both to power the device, and to recharge the device's internal battery. When such a device is connected to a host computer using a Universal Serial Bus (USB) interface, for example, the host computer transmits power to the device during exchange of data. This transmitted power may be used for the device operation, and also for recharging its battery. Many devices today allow for fast recharging of batteries when the devices are connected to an external power source, even while the devices are operational.
Cell phone batteries and other device batteries are generally lithium ion batteries. Lithium ion batteries offer several advantages. They have high energy-to-weight ratios, they have no memory effect, and they have slow discharge when not in use. In this regard, reference is now made to FIG. 1, which is a prior art graph of discharge of a lithium-ion battery over time. The graph in FIG. 1 is taken from http://www.ohararp.com/files/BUCK_BOOST_ARTICLE.pdf.
Discharge of a lithium ion battery or battery pack needs to be carefully monitored to prevent the end-of-discharge voltage from dropping below a safety level of 2.5V per cell. If allowed to self-discharge below 2.5V, a lithium ion battery undergoes irrecoverable capacity loss if it is stored in such condition for three months or longer. A protection circuit intrinsic to lithium ion battery packs prevents the batteries from being discharged below the safety level.
Cell phones and other such communication devices characteristically require high power bursts to perform certain operations. Cellular modems, for example, require series of short, high power bursts to achieve time division multiplexing. Due to these high power burst requirements, such a communication device may not be operable when using power received from a computer host, since the high power bursts may not be supported by the host, or may result in damage to the host. For such a communication device, external power received from a host may only be used to recharge the device's internal battery, but not for operation of the device.
Thus when a communication device's battery is low, the device can be connected to a host device to receive power for battery recharge, but cannot run on the host power supply. As such, the communication device cannot begin operating immediately after connection to the host, but must wait until the internal battery is sufficiently charged. Such wait may take several minutes. A cell phone, for example, which was shut down due to low battery charge, does not begin operating right away when it is connected to an external power source.
It would thus be of advantage to manage power of cell phones and other electronic devices, so as to enable turning on the cell phone, at least for emergency use, after it is shut down.